Once a ghost (that isn't a fear illusion, since those operate on different rules) is visible to the audience then it's possible for the characters to see it. The ghost here was visible as soon as it showed up in this chapter, but the characters didn't notice it right away because it was hiding behind a tree and in the grass.

There are some exceptions: the green ghost from chapter 1, and ghosts appearing to people who aren't ghost hunters. The green ghost can walk around visible to the audience yet invisible to the characters, because he's a freak important in ways that haven't been revealed yet.

People who aren't ghost hunters/don't know that ghosts are real are less likely to notice ghosts when they appear. It's actually a little more complicated than "if u no the secret then u see it all!!1!" but at the moment that's the best explanation.

So, does the size and look of the ghost reflect the age of the person? Or can it be used as a misdirection? That being said can ghosts change their appearance at will, or do they only take their human forms and ghost forms?

Good questions ahah, the ghost's appearance vs identity is addressed on the next page, and shape shifting in general is addressed in the next chapter. Every ghost has a number of different forms they take on naturally, and the farther they stray from those forms the harder it is to maintain.

Something about that hair curl and the colour tell me that this ghost can be planed. Then they could grow a ghost tree, and get ghost wood, and use it to make ghost traps with ghost bait of... ghost furniture?

*sighs* this is why every group needs a mad scientist. If you limit yourself to "sane" and "ethical" and all that nonsense then you'll never even know what you are missing out on!